This invention relates generally to a magnetic bubble memory element having a soft magnetic material pattern on a conductor pattern through an inter-laminar insulation layer, and more particularly to an inter-laminar insulation layer to obtain an excellent bubble transfer property in a high density element using a bubble having an extremely small diameter of below 1.2 .mu.m.
In the magnetic bubble memory element, the soft magnetic material pattern is formed on a bubble garnet film and a rotating magnetic field is applied in an in-plane direction to generate magnetic poles at the end portions of the pattern. Bubble transfer is made by use of these poles. On the other hand, the soft magnetic material pattern is formed on the conductor pattern through the inter-laminar insulation film. Therefore, if the level difference of the conductive pattern is transferred as such to the soft magnetic material pattern, the level difference occurs in the soft magnetic material pattern, too, and magnetic poles are generated in the portion having the level difference by the rotating magnetic field or by a bias magnetic field in a vertical direction. The poles in this level-difference portion exert adverse influences upon the bubble transfer, thus deteriorating transfer characteristics.
In order to solve this problem, a method has been proposed, as disclosed in Japanese Patent Laid-Open No. 22293/1980, for example, wherein a thermosetting resin is used as the inter-laminar insulation film between the conductor pattern and the soft magnetic material. This method comprises coating with a thermosetting resin solution and thermosetting the resin to form a resin insulation film. This method can be thought to be an extremely simple flattening method having high reproducibility because it can flatten the level difference of the conductor pattern by utilizing the fluidity of the thermosetting resin solution.
When a transfer path for ultrafine bubbles having a diameter of below 1.2 .mu.m is formed by this method, however, the transfer characteristics are found to be unsatisfactory. In other words, it has been clarified that the transfer characteristics drop in the level-difference portion of the conductor pattern in this case.
FIG. 3 of the accompanying drawings is a diagram showing the relationship between the transfer characteristics in this level-difference portion and the bubble diameter. Each of the samples used to determine this relationship was obtained by forming a first insulation film, a conductor pattern, a second insulation film and a soft magnetic material pattern on a magnetic film having a respective bubble diameter. Polyimide isoindoloquinazolinedione belonging to the group of polyimide resins was used as the second insulation film. It can be understood from the diagram that a transfer margin (the range of a bias magnetic field necessary for the smooth transfer of the bubble) is at least 10% for bubbles having a diameter of greater than 1.2 .mu.m and drops drastically when the bubble diameter is below 1.2 .mu.m. In other words, the transfer margin of at least 10% is necessary in order to attain a satisfactory memory operation.
It has thus been clarified that the smaller the bubble diameter, the more insufficient the flattening effect in accordance with the prior art method. It may be assumed that when the bubble diameter is reduced, a bias magnetic field necessary for permitting the existence of the bubble increases, so that a great unnecessary magnetic pole occurs with respect to only a limited level difference of the soft magnetic material pattern.
In conjunction with the level difference that causes the unnecessary magnetic pole, an inclination angle (.theta.) at the end portion of the conductor pattern, shown in FIG. 4 of the accompanying drawings, is more important than the extent of the level difference itself. In other words, magnetization inside the soft magnetic material tends to have a distribution as parallel and as continuous as possible due to the exchange interaction. For this reason, the magnetization inside the soft magnetic material in the level-difference portion described above faces substantially in a direction of the inclination angle. It is therefore believed that the greater the inclination angle (.theta.), the more the unnecessary magnetic pole. Additionally, the influence of the bias magnetic field tends to direct the direction of magnetization in the level-difference portion further towards the vertical direction so that the vertical component of magnetization increases with an increasing bias magnetic field and hence, the unnecessary magnetic pole increases, too.
FIG. 5 is a diagram showing the relationship between the inclination angle (.theta.) necessary for securing the transfer margin of 10% and the bubble diameter. The samples used to determine this relationship were the same as those used in FIG. 3, but the second insulation film was formed. It can be understood from FIG. 5 that the inclination angle must be from 20.degree. to 0.degree. when the bubble diameter is as small as below 1.2 .mu.m, in order to secure the transfer margin of 10%.